corrosion fundamentals

Corrosion Control and Prevention

Course Overview

• Why

• Frequency

• No Pressure Washer

• Pressure Washer

• Cleaning Materials

• CPC’s

• The Cost of Corrosion

• Types of Corrosion

• Factors of corrosion

• Stopping & Preventing

Fundamentals1Aircraft Wash Procedures2

Course Overview

• Inspection Frequency

• Inspection Procedures

• Inspection Methods

• Corrosion Prone Areas

• Preventive Maintenance

• Fastener Sealing

• Sealing Techniques

• Recommended Materials

Preventive Maintenance3

Inspection & Repair4

Course Objectives

Why is prevention important

What are the different types of corrosion

What are some mechanical factors

How to stop & prevent corrosion

Corrosion Refresher

Why is corrosion

prevention important

Corrosion Basics

Factors influencing corrosion

Types of corrosion

How to prevent

corrosion

Aviation Corrosion Preventionand Control Policy

CORROSION PREVENTION AND CONTROL PROCEDURES

are in compliance with TM 1-1500-328-23

(Aeronautical Equipment Maintenance Management

Policies and Procedures)

IF THERE IS NO CPC PROGRAM ESTABLISHED,

AIRCRAFT WILL BE SCHEDULED CPC

INSPECTION ON A 90-CALENDAR-DAY

INTERVAL

1995 1996 1997 1998 1999 2000 2001

Direct cost of corrosion damage– Parts Cost

1995 1996 1997 1998 1999 2000 2001

Indirect cost of corrosion damage– Man-Hours Cost

1995 1996 1997 1998 1999 2000 2001

Corrosion Total Cost due to Corrosion

Total Corrosion Costs

Aloha Airlines Flight 243 – April 1988

Explosive decompression @ 24,000 feet

Airplane operated in coastal environment with exposure to salt and humidity

Root cause – Epoxy bond failure which allowed water to enter gap and start corrosion process

WITH PROPER MAINTENANCE BY TRAINED CREWS COST AND DAMAGE CAN BE REDUCED

PREVENT

Why study corrosion?

Corrosion Description

What is Corrosion?

Weakening of material due to:

Loss of cross-sectional area

Shattering of metal due to hydrogen embrittlement

Cracking of polymer due to heat or sunlight exposure

What is Corrosion?

A battery is an engineered form of controlled corrosion

By releasing electrical energy

Corroding the internal metal

Result of a material with its

Process are

Occurs because of the tendency of metals to to their natural states

Basic Chemical Definitions

Basic Elements Involved in the Corrosion Process

Conditions for Corrosion

A corroding metal

A metal driving corrosion

Electrical contact between the anode and cathode

Water, salt water, battery acid, etc

Conditions for Corrosion

Electrons flow to cathode

Negative ions from the electrolyte

combine with metal ions from the anode

Ion flow

Electron flow

Diagram of Corrosion in a Car Battery

A car battery contains three of the four required elements for corrosion:

The Anode, Cathode, and the Electrolyte

Anode + Cathode -

Electrolyte

Powering the light, or any other appliance, adds the fourth element - the Conductive Path - thus starting

the corrosion process

Conductive Path

Anode + Cathode -

Electrolyte

PittingPlating

Now, electrons flow from cathode to anode and ions will flow from the anode to the cathode resulting in a

pitting of the anode and a plating of the cathode

Electron flow

Ion Flow

Anode + Cathode -

Electrolyte

Conductive Path

Electrolyte

Re-plated

Deposits

removed

A car battery is designed to reverse this corrosive action once a reverse voltage is

applied from the car alternator

Alternator

Anode + Cathode -

Conductive Path

Electron flow

Acidity or alkalinity of the medium

Stability of the corrosion product

Biological organisms

Variation in composition of the corrosive medium

High Temperatures

OTHER FACTORS WHICH AFFECT A METAL’S TENDENCY TO CORRODE:

Elimination of any of the four conditions stops corrosion

Electrochemical Process

Elimination

REVIEW

Q. What are the four requirements for electrochemical corrosion?

A. Anode, cathode, electrolyte, conductive path

Q. Why is a separator commonly used between the anodic and cathodic cells of a battery?

A. Eliminates the conductive path & prevents the corrosion process. It interrupts the electron flow between the anode and cathode.

Excellent References

Six Common Types Of Corrosion

1 • Uniform Etch / General Surface

2• Pitting Corrosion

3 Galvanic / Dissimilar Metal

4 Crevice Corrosion

5 • Filiform Corrosion

6 Intergranular / Exfoliation

Seven Common Types Of Corrosion

1 • Uniform Etch / General Surface

2 • Pitting Corrosion

3 Galvanic / Dissimilar Metal

4 Crevice Corrosion

5 • Filiform Corrosion

6 Intergranular / Exfoliation

7 • Concentration Cell

Types Of Corrosion

Uniform Etch Corrosion

General Surface Corrosion

A bare metal surface is exposed to an electrolyte

Parts of the surface become anodes and others cathodes

Metal ions combine with air to form corrosion products

General Surface Corrosion

Cathode AreaAnode Area

ElectrolyteConductive Path

Uniform Etch & General Surface Corrosion

REVIEW

Q. When would general surface (uniform etch) corrosion be a concern?

A. Thinning of structure to the point of failure.

Q. What is a type of protection for general surface corrosion?

A. Application of protective coating (platting, painting, oxide coating such as cladding)

Pitting Corrosion

ANODE CATHODE

ELECTROLYTE CONDUCTIVE

Pitting Corrosion

REVIEW

Q. Why would pitting corrosion be much more prone to provoke a catastrophic failure than uniform corrosion generally does?

A. Pitting corrosion can form from many different corrosions. Once corrosion pits are started, they can be hidden and difficult to detect. Add stress for one reason or another, sharp pits led to the generation of cracks, which can lead to Stress Corrosion Cracking

Galvanic / Dissimilar Metal Corrosion

Galvanic corrosion occurs when:

– Two different metals are in contact and are in electrolyte

ELECTROLYTE CONDUCTIVE PATH

Anode is the metal that gives up electrons easily and corrodes fastest

Cathode accepts electrons and shows less corrosion

ANODE CATHODE

ELECTROLYTE CONDUCTIVE PATH

Area of dissimilar metal

contact

Area of dissimilar

metal contact

Cadmium plated steel

washer

Stainless Steel Screw

– Electrochemically dissimilar metals

– These metals must be in electrical contact

– The metals must be exposed to an electrolyte

For Galvanic Corrosion to occur,three conditions must be present:

Galvanic SeriesMetals grouped together have little tendency to produce galvanic corrosion

Coupling metals from different groups will result in accelerated corrosion

There are exceptions and the chart is a learning aid

GALVANIC CHART

GALVANIC GROUPING OF METALS

REVIEW

Q.Where is galvanic corrosion most likely to occur on an aircraft?

A.Anywhere different types of metal come in contact with each other, especially where moisture is present

Q. Is it a good practice to clean corrosion on a magnesium gearbox housing with a stainless steel brush? Why or why not?

A. No. This would introduce dissimilar metals to the component and set-up a very aggressive corrosion situation

Crevice Corrosion

CREVICE CORROSION OCCURS WHEN:

- Electrolyte enters gap between metals

Corrosion occurs on one metal

Corrosion is generally not easily seen

ANODECATHODE

ELECTROLYTE

CONDUCTIVE

Crevice Corrosion

MOISTURE HAS PENETRATED THE HINGE’S FAYING SURFACES

Crevice Corrosion

THE CIRCLED RIVET HAS BROKEN THE PAINT COATING AND IS ABOUT TO FAIL

NOTICE THE BULGING OR SEPARATION OF FAYING SURFACES

Crevice Corrosion

Crevice corrosion combined with dissimilar metal corrosion at this wing attachment point resulted in an

in-flight break-up of a Cessna 310

Crevice Corrosion

Full blown crevice corrosion in an otherwise very seawater resistant material

Crevice Corrosion Prevention

CREVICE CORROSION CAN BE REDUCED BY:

REVIEW

Q.What is a ‘faying surface’?

A.The metal that is covered in a lap joint or a metal-to-metal joint

Q. Where are some lap joints that are susceptible to corrosion on the aircraft you work on?

Occurs on metal surfaces which have organic coatings

Caused by the diffusing of oxygen and water through coating

Filiform Corrosion

Pigments and resins block surface to dissipate moisture

Moisture beneath paint film allows corrosion to form

Filiform Corrosion

FILIFORM CORROSION OCCURS WHEN:

- There is a break in the coating

- Corrosion grows under the coating

- Coating lifts as corrosion products form

CONDUCTIVE PATH

CATHODE

ELECTROLYTE

Typical Filiform Corrosion

Corrosionby-products cause paint or plating to lift off surface and bubble

Bubbled paint or plating

Because of bubbled appearance this can be confused for improper painting

Not coating failure

Filiform Corrosion Prevention

Store aircraft in low humidity environment (below 65% relative humidity)

Low humidity

Decrease corrosion by using coatings that have a high resistance to diffusion by water

Better quality coatings

REVIEW

Q. Where is filiform corrosion most likely to occur on an aircraft?

A. Under a dense coating of topcoat enamel such as polyurethane.

Q. What is the likely cause for filiform corrosion?

A. Poor preparation or improperly cured primers.

Q. Where are some areas that are susceptible to filiform corrosion on the aircraft you work on?

Intergranular Corrosion

Intergranular or exfoliation corrosion occurs when:

- Electrolyte attacks grain boundaries

- Corrosion products expand

- Layers of metal separate

CATHODE

ELECTROLYTE

A highly magnified cross section of any alloy shows the granular structure of the metal

The alloy has a large number of individual grains

Each tiny grain has a clearly defined boundary

The boundaries of the grains, chemically differs from the metal within the grain center

Frequently, the grain boundaries are anodic to the main body of the grain

When in contact with an electrolyte, rapid selective corrosion of the grain boundaries occurs

Exfoliation Corrosion

Exfoliation is an extreme form of

intergranular corrosion

The lifting up, or swelling is visible evidence of

intergranular corrosion and is most often seen on

extruded sections where grain thicknesses are usually

less than in rolled forms

Intergranular / Exfoliation Corrosion

Q.Where is intergranular corrosion most likely to occur on an aircraft?

A. Along the grain boundaries of aluminum alloys that have been improperly heat-treated. Extruded aluminum alloy is susceptible to intergranular corrosion.

Concentration Cell Corrosion

•Metal ion concentration cells1

• Oxygen concentration cells2

• Active / Passive cells3

TYPES OF CONCENTRATION CELL CORROSION

Concentration Cell Corrosion

Metal Ion Concentration Cells Corrosion

The area between the sheets contains a high concentration of positive metal ions, and it is

the cathodic area of the corrosion cell

Metal Ion Concentration Cells Corrosion

The area between the sheets contains a high concentration of positive metal ions, and it is the

cathodic area of the corrosion cell

Oxygen Concentration Cells Corrosion

The positive aluminum ions attract negative hydroxide ions from the water

and form corrosion

Oxygen Concentration Cells Corrosion

The positive aluminum ions attract negative hydroxide ions from the water and form corrosion

Active/Passive Cell Corrosion

Appears on metals that depend on a tightly adhering passive film for corrosion

protection, such as stainless steel

Starts when the passive film is broken by dirt or other foreign substances

deposited on the surface

Once the film is broken, the metal beneath exposed to attack and the result is rapid

pitting of the active metal

REVIEW

Q. What type of concentration cell corrosion forms between a lap joint in aircraft skin?

A. Oxygen Concentration Cell

Q. Where will you most likely see the formation of Metal Ion Concentration Cell corrosion?

A. At the edge of aircraft skin.

Q. Where are some components that are susceptible to Concentration Cell Corrosion on the aircraft you work on?

REVIEW

Q. What are the six common types of corrosion found on PGAG 18 aircraft?

A. Uniform etch/general surface, pitting, galvanic/dissimilar metal, crevice, filiform & intergranular/exfoliation.

Mechanical Factors to Corrosion

Stress Corrosion Cracking

Stress can be residual as a result of the production

process

The combined action of a

static tensile stress and corrosion

forms cracks

Stress may be a result of externally

applied cyclic loading

Stress corrosion cracking is caused by the

simultaneous effects of constant tensile stress

and corrosion

Internal stresses are produced by;

– Non-uniform deformation during cold working

– By unequal cooling from high temperatures

– By internal structural rearrangement involving volume changes

Stressed Corrosion Cracking (SCC) is the most subtle forms of corrosion because environmental cracks are microscopic in

their early stages of development

Pitting will greatly increase SCC

Crack will follow the grain structure

Crack driven by sustained and intermittent tensile stresses

Stress Corrosion Cracking Prevention

Prevention;

1) Reduce the overall stress in the initial design

2) Use suitable materials

3) Design the part to minimize residual stresses

4) Use suitable protective coatings

REVIEW

Q. Where is stress corrosion most likely to occur on an aircraft?

A. In a metal component that is continually under a tensile stress.

Q. Does the application of a part directly effect the susceptibility of Stress Corrosion Cracking?

A. Yes, high stress in corrosive environment will cause cracking sooner than high stress alone.

Fretting Corrosion

Caused by motion between two surfaces

Fretting Corrosion

Rapid corrosion occurs at the point of contact between the highly loaded metal surfaces when subjected to small vibrations

Fretting Corrosion

This may be an indicator

of motion between two tight

fitting parts

Fretting Corrosion Prevention

1) Design out vibration

2) Lubricate metal surfaces

3) Increase the load between the surfaces to stop the motion

4) Surface treatments to reduce wear and increase friction coefficient

Fretting Corrosion

Q. Where is fretting corrosion most likely to occur on an aircraft?

A. In a location where there is a slight amount of relative movement between two components and no way for the corrosive residue to be removed.

Q. What are some visual indications of fretting corrosion?

A. Smoking hinges or rivets, residue seeping from fayed surfaces after cleaning.

Q. Where are some location on the UH-60 or CH-47 where fretting is common?

Corrosion Fatigue

• Caused by combined effects of cyclic stress and corrosion

• No metal is immune

• Rate of decay is greatly increased by corrosive environments

• Damage is greater than the sum of the damage from both cyclic stresses and corrosion

Corrosion Fatigue

The fatigue limit in both stress and cycles of a metal is greatly reduced in a corrosive environment

Corrosion Fatigue

Fatigue fractures are brittle and cracks will follow metal grain structure

Corrosion Fatigue – Aloha Airlines Flt 243

Suffered an explosive

decompression

Lost a large part of it’s

upper fuselage while climbing through 24,000

One flight attendant was sucked out of

the aircraft and the cockpit

sagged down 3 feet

The aircraft had 35,496 hours TT and 89,680 cycles

History of cracks and corrosion

This aircraft had 24 fuselage skin repairs before the accident

Operated in a severe corrosion environment

The 1300 rivets to be inspected were painted

Night shift, boredom, fatigue and length of the inspection task

Inadequate work stands, lighting

Insufficient training to recognize corrosion fatigue

REVIEW

Q. What are the two stages in which corrosion fatigue occurs?

A. Material damage stage (due to corrosion and cyclic stress) and material fatigue stage (cracks spread).

True or False; once the corrosive environment is removed, the metal can be considered to meet original strength limits.

False, stage one damage has already happened. Material damage has occurred.

Hydrogen Embrittlement

Affects high strength steels

Caused by action due to plating

Hydrogen enters metal during plating process

Electroplating and/or

replating

Corrosion on high strength

fasteners

Use of chemical

cleaners and strippers

MAJOR CAUSES OF HYDROGEN EMBRITTLEMENT:

British Columbia – June 2000

Manufacture improperly heat-treated screws on FCU

Once cad-plated, hydrogen was introduced into the steel

Screw head separated from FCU cover

10 October 1992, near Fort Richardson, Alaska

Cause was failure of upper boost actuator fastener

Improper cleaning agent used (Simple Green)

Aircraft destroyed. Crew received minor injuries

How many pieces of aircraft hardware

are zinc or cadmium plated?

Q. What are some factors that cause hydrogen embrittlement?

A. High strength fasteners, plating processes, improper cleaning agents & chemical strippers,

Q. What are some areas/components that are prone to hydrogen embrittlement on Group 18 aircraft?

Factors Influencing Corrosion

Sea water is an aggressive corrosive

Electrolyte Concentration is one of the most important

factors influencing the rate of corrosion on Group 18

Aircraft

Contaminants like salt, atmospheric pollution, dirt, soda, increases the strength of the water electrolyte

The more concentrated the electrolyte the faster corrosion

occurs

Distilled Water Sea Water

Small anode toLarge cathode

FAST CORROSION

Large anode toSmall cathode

SLOW CORROSION

Steel Rivets (anodic) in Copper Bar (cathodic) at Start of Experiment

Copper Rivets in Steel Bar at Start

of Experiment

Steel Rivets in Copper Bar

Copper Rivets in Steel Bar

Steel Rivets in Copper Bar

Copper Rivets in Steel Bar

Determines speed of corrosion:•Most anodic metal corrodes

first• The more dissimilar the metal,

the faster the corrosion

Galvanic Chart:• Shows corrosive potential of

dissimilar metals• Show reactivity of metals

GALVANIC CHART

REVIEW

Q. What are some main factors that influence the rate of corrosion?

A. Electrolyte concentration, humidity, ambient temperature, anode to cathode ratio, galvanic difference between metals

Q. What would corrode fastest in identical atmospheric conditions: steel rivets in a copper bar or copper rivets in a steel bar?

Stopping and Preventing Corrosion Basics

Electrons flow to cathode

Ion Flow

Electron Flow

Negative ions from the electrolyte combine with

metal ions from the anode

Controlled by the DesignersThe materials used in the aircraft were selected by engineers

Some of these materials react with other materials

Do not accelerate corrosion by introducing a galvanic difference

Don’t use unapproved hardware

Eliminate the conductive path by:

Using primer, sealant, & dry film lube

Wet fastener installation

Sealing faying/fretting surfaces

Considerations:

Flexing of joints

Electrical bonding

Installation & removal requirements;

• Inspection

• Replacement

Removal may be accomplished by:

Clearing drain holes

Regular washing of the aircraft

Drying the aircraft

Primers, sealants, greases and dry film lubricants that contain corrosion inhibitors should always be used

Inhibitor acts like small ‘anode’ embedded in the CPM

Corrosion Preventative Materials (CPM’s)

CPM is sacrificial and will “corrode” first

Effect is only temporary and lasts for short period of time

Repaint, reseal, reapply when CPM is damaged or deteriorated

Corrosion Preventative Materials (CPM’s)

Inhibitors in the Primer provide sacrificial protection to the metal substrate when coating becomes cracked or damaged

Primer

Top Coat

Sacrificial elements

suspended in the coating

Breaks or defects in coating

Removing any one of the four elements of the battery model, will stop the advancement of corrosion and prevent further

corrosion from starting

Of the four parts of the battery model, the easiest to remove is

the electrolyte.

It is also the most difficult to keep removed because of

weather

Understanding what corrosion is and how it works is the first step in combating corrosion

REVIEW

Q. What are some ways to remove the conductive path on a gearbox installation?

A. Install fasteners ‘wet’, use approved corrosion inhibitors & seal the faying surfaces between casing and mount.

Q. What are some precautions to be aware of when eliminating the conductive path?

A. Flexibility, electrical bonding, frequency of removal/installation.

REVIEW

Q. How can a technician minimize the galvanic corrosion caused by anodes and cathodes in an aircraft?

A. Use correct hardware during all phases of maintenance and prevent accumulation of debris inside the aircraft.

Q. What are some ways to remove the electrolyte from your current aircraft?

A. Clean out all drain holes then wash & dry the aircraft, that is the best defense.

Corrosion Refresher

Q. Why is it important to understand the different types of aircraft corrosion?

Q. What types of corrosion have been found on your current fleet of aircraft?

Q. What are some of the ways general surface corrosion can be reduced?

Corrosion Refresher

Q. What type of corrosion forms in riveted joints where there is a slight relative movement?

Q. What is an extreme case of intergranular corrosion that forms in extruded metal?

Q. What is another name for ‘dry corrosion’?

Corrosion Refresher

Q. Why is it important to understand the different types of aircraft corrosion?

A. To minimize corrosion damage so that maintenance costs are reduced and incidents are eliminated.

Q. What types of corrosion have been found on your current fleet of aircraft?

A. Almost all kinds: Surface, galvanic and pitting are most common.

Corrosion Refresher

Q. What are some of the ways general surface corrosion can be reduced?

A. Stop movement of electrons, stop oxygen from reaching surface, use sacrificial coating, use sacrificial metal.

Q. What type of corrosion forms in riveted joints where there is a slight relative movement?

A. Fretting.

Corrosion Refresher

Q. What is an extreme case of intergranular corrosion that forms in extruded metal?

A. Exfoliation, it separates the metal in layers.

Q. What is another name for ‘dry corrosion’?

A. Oxidation or uniform etch corrosion.

Corrosion Refresher

Q. What type of corrosion is prevented by maintaining intact sealants?

A. Concentration cell corrosion.

Q. What is Stress Corrosion Cracking?

A. A crack formed by the combined action of stress and a corrosive environment. The crack would not have developed by the action of the stress or environment alone.

Corrosion Refresher

Q. Why is Stress Corrosion Cracking a serious safety problem?

A. Because it can happen ‘unexpectedly’ and rapidly after a period of satisfactory service, leading to catastrophic structural failure.

Q. What are the two stages of corrosion fatigue called?

A. Material damage stage (due to corrosion and cyclic stress) and material fatigue stage (cracks spread).

Publications Used for Information

TECHNICAL MANUAL TITLE

TM 1-1500-344-23-1Cleaning and Corrosion ControlVolume 1Corrosion Program and Corrosion Theory

TM 1-1500-344-23-2Cleaning and Corrosion ControlVolume IIAircraft (Cleaning, Inspection, Repair, Sealants, Preservation)

TM 1-1500-344-23-3Cleaning and Corrosion ControlVolume IIIAvionics and Electronics

TM 1-1500-344-23-4Cleaning and Corrosion ControlVolume VIConsumable Materials

TM 1-1500-344-23-5Cleaning and Corrosion ControlVolume VConsumable Materials for Avionics

TM 1-1500-204-23 General Aircraft Maintenance and Practices

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